Composition for, and method of applying coatings of polychlorotrifluoroethylene



COIVIPOSITION FOR, AND METHOD OF APPLY- ING COATINGS OFPOLYCHLOROTRIFLUORO- ETHYLENE Mortimer H. Nickerson, Hazardville, Conn.

No Drawing. Application January 11, 1957, Serial No. 633,516

14 Claims. (Cl. 117-65) This invention relates to the formation ofcoatings of polychlorotrifluoroethylene.

This application is a continuation in part of application Serial No.330,350, filed January 8, 1953, now abandoned.

Polychlorotrifiuoroethylene in high molecular weight form has aremarkable resistance to attack by corrosive materials and also byorganic solvents. ln addition, it possesses excellent electricalinsulation properties which are enhanced by its ability to withstandhigher temperatures than most known plastics, coupled with a negligiblewater absorption. This combination of properties makespolychlorotrifluoroethylene almost ideal as a protective coating toprevent chemical or solvent attack or to prevent electrical leakage onconductors. The use of this polymer in the above manner has, however,been severely handicapped by the difliculty of applying such coatings.Since there is no known solvent which will dissolve appreciable amountsof the polymer at room temperature, the material cannot be applied as alacquer as is commonly done with most protective coats.

One approach to the problem is to make dispersions of the finely groundpolymer in a suitable volatile medium and thus obtain a thin coat of thematerial by first applying a coat of the dispersion, evaporating thedispersing medium, followed by fusion at sufiiciently high temperatureto cause the fine particles to melt and coalesce to a continuous film.Hitherto, work along this line has not been very successful, due largelyto the fact that unless the dispersion is applied in an extremely thincoat the powder layer which is left on evaporation of the dispersingmedium shrinks and cracks prior to fusion. This type of cracking isquite common in most dispersion work and is sometimes described as mudcracking. if this cracking takes place, the resulting film has manydiscontinuities and therefore has no value for protection or elec tricalinsulation. The extremely low value for the maximum thickness which canbe applied by the above procedure in any one coating operationnecessitates multiple coating operations in order to build up asubstantial film. Prior to the present invention, it was necessary toperform three or four successive coating and fusing operations to buildup as little as 0.001 inch thickness of protective polymer film. Theseoperations are not only laborious, but also successive heating of thepreviously laid down layers of polymer tends to weaken them physically,leading in extreme cases to serious embrittlement of the resin.

It is the object of this invention to provide dispersions ofpolychlorotrifluoroethylene by means of which it is possible to apply inone operation a coating of the polymer having a final fused thickness upto 0.005 inch or higher, such coating consisting of a well-adhered,continuous film, free of cracks. A further object of this invention isto provide techniques and methods by means of which such coatings areobtained.

The first requisite in making a dispersion of any solid material is thatthe material be finely ground. Poly- States PatentO 2chlorotrifluoroethylene suitably prepared in a finelydivided state byany of the presently known methods may be used but for obtaining theultimate in glossiness and smoothness of a coat, it is necessary toreduce the basic particle size by conventional equipment such as amicropulverizer. *If a slightly granular coat will serve the purpose, itwill usually be sufficient to take the polymer powder as commerciallysupplied and break up any aggregates, as can be done, for example, bysuspending the material in a suitable liquid medium and passing itthrough a colloid mill or by ball-milling. It should be borne in mindthat the particle size of the polymer mainly alfects the'glossiness ofthe product and does not affect the cracking of the film which it is theobject of this invention to eliminate. :In general the finer theparticle size, the glossier the product. As previously statedcommercially available high molecular weight polychlorotrifluoroethylenecan be used satisfactorily provided that it has been pulverized by anyof the usual means to reduce it to' the fine particle size necessary fordispersions. In general this size is in the range of l to 20 microns.

My investigation and study of the process of dispersion coating usingpolymeric chlorotrifuloroethylene has led to the conclusion that thephenomenon of mud cracking takes place after the evaporation of thevolatile dispersing medium and before fusion of the polymer, due toheat, takes place. I have found that the conventional choice of alow-boiling dispersing medium only aggravates the conditions which causemud cracking; because of the greater spread in time and temperaturebetween the disappearance of the last of the dispersing medium and thefusion of the polymer. According to the present invention the idealdispersing medium should have a boiling point that will allow thesimultaneous evaporation of the dispersing medium, fusion of the resin,and attainment of the fusion temperature by the article being coated.Under these conditions the intermediate dry condition where all of thedispersing medium has evaporated and the polymer exists as a weak layernever occurs and discontinuities due to shrinking of the weak layer willnot result.

In addition to having a high boiling point I have found that thedispersing medium must possess other properties. One of these properties-I describe as paste forming. By this I mean that it possesses theability to form smooth relatively thick, continuous, sustained films ofthe dispersed polymer at below fusion temperature. Other requirementsare that the medium must be chemically stable at rather hightemperatures so that it will volatilize cleanly from the polymer film asit fuses. Preferably, it should exert some solvent action on thepolymer, at least at high temperature, to facilitate fusing of the film,and necessarily it must be at least partially soluble in the polymer inorder that it may escape from the fusing film without bubbling. Themedium must not chemically interact with the polymer to change itsproperties or promote decomposition. Obviously if the dispersion is tobe applied at room temperature, the dispersing medium must be a liquid.

' I have found that the chlorinated diphenyls containing 18% to 62% ofcombined chlorine and boiling in the range 225 C. to 430 C. meet theunique combination of properties described above and when used as thedispersing medium for polychlorotrifluoroethylene make possible theproduction of continuous, crack-freefilms of fused polymer of athickness hitherto impossible to attain.

Chlorinated diphenyls suitable for this work are available under thetrade name Aroclor from the Monsanto Chemical Company. There is anextensive series of these materials, and they are available in a varietyof chlorine contents, molecular weights, and boiling points Theparticular chlorinated diphenyl to be used will depend upon thecross-sectional dimensions of the article to be coated. That member ofthe series designated as Aroclor 1221, which has a distillation range of275 C. to 320 C., is preferred as the dispersing medium for work on thincross-sections, particularly on thickness'less' than 0.125 inch. Thehigher boiling members of the series are preferred for thickercross-section articles. It was found that a steel bar of 1 inchthickness could be successfully coated with-polychlorotrifluoroethyleneusing that member of the series designated as Aroclor 1262, which has adistillation range of 400 C. to 430- C. It is not necessary that onedefinite chlorinated diphenyl be the sole constituent of the dispersingmedium, but rather, mixtures of chlorinated diphenyls can be used.Similarly, other more volatile liquids may be used as diluentsinconjunction with the chlorinated diphenyls in order to reduce viscosity.In such cases the essential feature is that sufficient chlorinateddiphenyl be used so that after evaporation of the more volatileconstituents, there is present enough chlorinated diphenyl to hold theresin in a continuous film by means of its paste-forming ability asdescribed above. I have found that in general an amount of chlorinateddiphenyl equal to 50% of the dispersed polymer is adequate to maintainthis continuity of film prior to and during fusing, although the use ofa larger amount is preferable.

Requirements for the diluent are much less stringent since its onlyfunction is to provide fluidity to the dispersion. However, it should bemiscible with the chlorinated diphenyl and should also act as a gooddispersant for the polymer. I have found that a mixture of solvents isthe best diluent and that for best results it should consist of threeparts of an aromatic hydrocarbon of the class benzene, toluene, xyleneand one part of an aliphatic ketone of the class acetone, methyl ethylketone, methyl isobutyl ketone, di-isobutyl ketone. For general use themixture of xylene and methyl isobutyl ketone is preferred, but othercombinations may be used depending upon what volatility is most suitedfor the application at hand.

The following examples are ofiered by way of illustrating how theteachings of this invention may be used;

Example l.--Polychlorotrifiuoroethylene, micropulverized to a particlesize of about three microns is added gradually with stirring to achlorinated diphenyl having a chlorine content of about 21% and boilingin the range 225 C. to 350" C. S'ufficient polymer is so added that thefinal weight of polymer is equal to the weight of chlorinated diphenyl.The resulting-product is a smooth, soft paste which can readily bespread and leveled to a smooth coat.

A portion of this paste was spread on a flat steel plate toa depth ofapproximately 12 mils and the plate then heated rapidly to a temperatureof 250- C. and kept at that temperature for three to five minutes oruntil the fumes of chlorinated diphenyl ceased to come olf thefilm'. ncooling the plate, the polymer was found to bein the form of a clear,transparent film, free of cracks, and tightly adhered to the plate. Thethickness was 5 mils.

Example HLA portion of the paste prepared as in Example I was dilutedwith a mixture of three parts xyleneand one part methyl isobutylketone,using one part of the mixture to onepart of the paste. The paste readilystirred into the diluent to produce a free-flowing, low-viscositydispersion. A puddle of this dispersion was poured? onto a flat steelplate and then flowed over the surface by tilting the plate. The coatedplate" was then dried briefly, at a temperature of 1200" C'.,to-ridi'tat least partially of the lower boiling diluent. The plate" wasthen heated rapidly to'a temperatureof' 250 C. and" kept at thattemperature" for three to fiveminutes' or' until fumes-of thechlorinated diph'enyl ceased to'come off the film. On cooling the plate,the polymer'wasifound' to be in: the form of a clear, transparent film,free of cracks and tightlyadhered to the-plate: The thickness was 2.5mils.

Example III.-A paste prepared as described in Example I was diluted withan equal part by weight of chlorinated diphenyl of the same type used inmaking the paste. There resulted a low-viscosity fluid dispersion whichflowed readily over the surface of a metal plate to produce a uniformcoating. This coating was then fused by the same procedures as used inExamples I and II without the necessity of an intermediate drying step.The resulting coating was clear and transparent, tightly adhered to themetal, and had a thickness of. 2 mils.

Example IV.-A dispersion was prepared by mixing the followingingredients in a ball-mill, colloid mill, or other homogenizing device:

parts finely divided polychlorotrifluoroethylene 55 parts Aroclor 122175 parts xylene 25 parts methyl isobutyl ketone (By weight) Theresulting product is a smooth white liquid mass exhibiting somethixot'ropic characteristics. A portion of this dipersion was spread onaflat metal plate to a depth of about 15 mils and then warmed to 100 C.for live minutes to volatilize most of the diluent. The plate was thenrapidly heated to 250 C. and fused as described in the precedingexamples. The resulting coating was clear, transparent, free of cracks,'and tightly adhered to the metal plate. Its thickness was 4 mils.

Example V.--A dispersion was prepared from the following ingredients asdescribed in Example IV.

133 parts finely divided polychl'orotrifiuoroethylene 122.5 partsAroclor' 1262 4l parts Aroclor 1221 37 parts methyl is'obutyl ketone Theresulting product is a smooth white pasteexhibiting thixotropicproperties. A portion of this dispersion was spread on a clean steel-barof the dimensions 1 inch by 3 inches by 10 inches to a depth of. about 7mils. The bar was rapidly heated to 250 C- for 50 minutes followed by acold water quench; The resulting. coating was clear, transparent, freeof cracks, and tightly adhered to the metalbar. Its thickness was 2.5mils-.1

It will be readily seen that these dispersions are extremely versatileandcan be used in: a number ofways. The heavier pastes are suitable forknife coating. and, by dilution with the indicated thinners oradditional chlorinated diphenyl, are suitable for spraying, casting}flow coating, or dipping. The dispersions can be used to coat metal,glass, ceramics. wire, smooth: or irregularly shaped objects. The onlybasic requirement of the objectto be coated is that it must withstandwithout objectionable decomposition the temperatures necessary to fusethe polymer.

Within the general classification of high molecular weight polymers;there are available" commercially a number of grades differing to someextentinmolecular weight and correspondingly in softening point. Aroclor1221 has: a boiling range making it useful for a fairly large groupingof these grades of polychlorotrifluoroeth- 'ylene, but when higher orlower molecular weight polymers are used, itmay be preferable to use achlorinated d'iphenyl having a correspondingly higher or lowerboilingrange in order to preserve the proper relationship betweenboiling range of' the Aro'clor and the softening point of the resin. Thechoice of a chlorinated diphenyl is therefore not restricted to Aroclor1221', and the use of any other such material according to the aboveteachings is within the scope of this invention;

What is claimedis: 7

1. A coating composition a dispersion of finely dividedpolychlorotrifluoroethyl'ene in a chlorinated diphenyl containing 20% to40% of combined chlorine having a boiling range essentially includingthe fusion temperature of the polymer and present in a quantity at leastapproximately equal to 50% by weight of the dispersed dry polymer.

2. A coating composition comprising a dispersion of finely dividedpolychlorotrifluoroethylene in a dispersion medium the essentialconstituent of which is a chlorinated diphenyl containing 20% to 40% ofcombined chlorine, boiling in the range 225 C. to 350 C. and present ina quantity at least approximately equal to 50% by weight of thedispersed dry polymer to maintain the continuity of the coating filmprior to and during fusion of the polymer.

3. A coating composition as in claim 2 in which the chlorinated diphenylis present in an amount in excess of 50% by weight of the dispersedpolymer.

4. A coating composition comprising a dispersion of finely dividedpolychlorotrifluoroethylene in a dispersion medium the essentialconstituent of which is a chlorinated diphenyl containing 20% to 40% ofcombined chlorine, having a boiling point in the range 225 C. to 350 C.and present in a quantity at least approximately equal to 50% by weightof the dispersed dry polymer, the balance of the dispersion mediumcomprising a diluent miscible with the chlorinated diphenyl andevaporating without decomposition at a temperature below the fusiontemperature of the polymer.

5. A coating composition as in claim 4 in which the diluent constituentof the dispersion medium comprises a mixture of 3 parts by weight of anaromatic hydrocarbon of the class benzene, toluene, xylene, and 1 partby weight of an aliphatic ketone of the class acetone, methyl ethylketone, methyl isobutyl ketone, and diisobutyl ketone.

6. A coating composition as in claim 4 in which the diluent constituentconsists of 3 parts by weight Xylene and -1 part by Weight methylisobutyl ketone.

7. A coating composition comprising a dispersion of finely dividedpolychlorotrifluoroethylene in a chlorinated diphenyl containing 20% to40% of combined chlorine, boiling in the range 225 C. to 350 C. andpresent in an amount not substantially less than the dry polymer byweight.

8. A coating composition comprising a dispersion of finely dividedpolychlorotrifluoroethylene in a chlorinated diphenyl containing 18% to62% of combined chlorine, boiling in the range 225 C. to 430 C. andpresent in an amount not substantially less than the dry polymer byweight.

9. A coating composition comprising a dispersion of finely dividedpolychlorotrifluoroethylene in a dispersion medium the essentialconstituent of which is a chlorinated diphenyl containing 18% to 62% ofcombined chlorine, boiling in the range 225 C. to 430 C. and present ina quantity at least approximately equal to 50% by weight of thedispersed dry polymer, the balance of the dispersion medium comprising adiluent miscible with the chlorinated diphenyl and evaporating withoutdecomposition at a temperature below the fusion temperature of thepolymer.

10. The method of providing articles with a continuous crack-freecoating of polychlorotrifiuoroethylene which comprises dispersing finelydivided polychlorotrifluoroethylene in a paste forming medium having aboiling range within a range including the fusion temperature of thepolymer, both chemically stable and at least partially soluble in thepolymer at said temperature, applying said paste dispersion over thesurface of the article to be coated, heating the coated surface to thefusion temperature of the polymer to substantially simultaneouslyevaporate the dispersing medium completely and fuse the polymer, andcooling the article.

11. The method of providing articles with a continuous crack-freecoating of polychlorotrifluoroethylene which comprises dispersing finelydivided polychlorotrifluoroethylene in a dispersion medium essentialconstituent of which is a chlorinated diphenyl containing 20% to 40% ofcombined-chlorine having a boiling point in the range 225 C. to 350 C.and present in a quantity at least equal to 50% by weight of thedispersed dry polymer, the balance of the dispersion medium comprising adiluent miscible with the chlorinated diphenyl and evaporating withoutdecomposition at a temperature below the fusion temperature of thepolymer, applying said dispersion over the surface of the article to becoated, raising the temperature of the surface of the article to a pointto at least partially evaporate the di: luent, then rapidly raising thetemperature of the surface of the article to the fusion temperature ofthe polymer to substantially simultaneously evaporate the chlorinateddiphenyl and fuse the polymer, and finally cooling the article.

12. The method recited in claim 11 in which the diluentconstituent ofthe dispersion medium comprises a mixture of 3 parts by weight of anaromatic hydrocarbon of the class benzene, toluene, xylene, and 1 partby weight of an aliphatic ketone of the class acetone, methyl ethylketone, methyl isobutyl ketone, and di-isobutyl ketone.

13. The method recited in claim 11 in which the diluent constituentconsists of 3 parts by weight xylene and 1 part by weight methylisobutyl ketone.

14. The method recited in claim 11 in which the essential constituent ofthe dispersion medium is a chlorinated diphenyl containing 18% to 62% ofcombined chlorine having a boiling point in the range 225 C. to 430 C.

2,542,069 Young Feb. 20, 1951 Compton June 6, 1950

10.THE METHOD OF PROVIDING ARTICLES WITH A CONTINUOUS CRACK-FREE COATINGOF POLYCHLOROTRIFLUOROETHYLENE WHICH COMPRISES DISPERSING FINELY DIVIDEDPOLYCHLOROTRIFLUOROETHYLENE IN A PASTE FORMING MEDIUM HAVING A BOILINGRANGE WITHIN A RANGE INCLUDING THE FUSION TEMPERATURE OF THE POLYMER,BOTH CHEMICALLY STABLE AND AT LEAST PARTIALLY SOLUBLE IN THE POLYMER ATSAID TEMPERATURE, APPLYING SAID PASTE DISPERSISON OVER THE SURFACE OFTHE ARTICLE TO BE COATED, HEATING THE COATED SURFACE TO THE FUSIONTEMPERATURE OF THE POLYMER TO SUBSTANTIALLY SIMULTANEOUSLY EVAPORATE THEDISPERSING MEDIUM COMPLETELY AND FUSE THE POLYMER, AND COOLING THEARTICLE.